Window Live Movie Maker - video footage audio to soundtrack audio

HELLLLP - I've looked everywhere and I cannot find an answer to this question! I am﻿ clear on how to replace the video footage audio with a soundtrack (thank you!) However, HOW do you go back and forth from video footage audio to soundtrack audio within the movie? I have a soundtrack playing during one portion of my movie - then I want to fade it out to listen to a speaking part within the movie and I can't do it - HELPP!